This application is based on Japanese Patent Application No. 2001-167511, of which content is incorporated by reference.
1. Field of the Invention
The present invention relates to a liquid crystal composition and a reflective type liquid crystal display which uses the liquid crystal composition.
2. Description of Related Art
A liquid crystal display generally comprises a pair of substrates with transparent electrodes thereon and a liquid crystal layer filled between the pair of substrates. By applying a driving voltage to the liquid crystal, the alignment of the liquid crystal molecules is controlled so as to modulate external light which is incident to the display, and thereby, a desired image can be displayed.
Various methods of writing an image on liquid crystal have been suggested. In recent years, various kinds of liquid crystal displays which use chiral nematic liquid crystal compositions have been studied, the chiral nematic liquid crystal compositions being produced by adding chiral agents to nematic liquid crystal and exhibiting a cholesteric phase at room temperature.
Such a liquid crystal display is known, for example, as a reflective type liquid crystal display which uses selective reflection of the liquid crystal at the cholesteric phase and which accordingly has an advantage of consuming little electric power. In such a reflective type liquid crystal display, pulse voltages with high energy and with low energy are selectively applied to the liquid crystal, and thereby, the liquid crystal is switched between a planar state (colored state) and a focal-conic state (transparent state). In this way, an image is written on the display. Then, after the application of the pulse voltage is stopped, the liquid crystal stays in the planar state, the focal-conic state or an intermediate state therebetween. (This characteristic of the liquid crystal of staying in the planar state or in the focal-conic state is generally referred to as bistability or memory effect). Thus, the image written on the display can be continuously displayed even after stoppage of the voltage application.
Also, in order to permit full-color display, it is possible to structure such a liquid crystal display into a three-layered display composed of an R liquid crystal layer for display of red, a G liquid crystal layer for display of green and a B liquid crystal layer for display of blue.
Such reflective type liquid crystal displays which use chiral nematic liquid crystal, however, have a demerit that because high driving voltages are necessary to drive these displays, inexpensive ICs for general purpose cannot be used. The reason is described. Chiral agents of which anisotropy of dielectric constant is small, that is, not more than 10 have been used for chiral nematic liquid crystal. When such a chiral agent with a small anisotropy of dielectric constant is added to nematic liquid crystal so as to produce a chiral nematic liquid crystal composition which exhibits a cholestric phase at room temperature, the anisotropy of dielectric constant of the chiral nematic liquid crystal composition is too small, and it follows that a high voltage is necessary to drive the chiral nematic liquid crystal composition.
An object of the present invention is to provide a liquid crystal composition and a reflective type liquid crystal display which require low driving voltages and which can use inexpensive ICs for general purpose.
The inventors had been studied so as to achieve the object, and they found that by adding a chiral agent with a relatively high anisotropy of dielectric constant to a nematic liquid crystal mixture, the obtained chiral nematic liquid crystal composition has a higher anisotropy of dielectric constant than that of the nematic liquid crystal mixture or has such an anisotropy of dielectric constant as to permit a drive of the chiral nematic liquid crystal composition by application of a low voltage.
A liquid crystal composition according to the first aspect of the present invention is a chiral nematic liquid crystal composition which contains a nematic liquid crystal mixture and a chiral agent, which exhibits a cholesteric phase at room temperature and which selectively reflects light of a specified wavelength, and the anisotropy of the dielectric constant of the chiral nematic liquid crystal composition is larger than that of the nematic liquid crystal mixture before being mixed with the chiral agent.
A liquid crystal composition according to the second aspect of the present invention is a chiral nematic liquid crystal composition which contains a nematic liquid crystal mixture and a chiral agent, which exhibits a cholesteric phase at room temperature and which selectively reflects light of a specified wavelength, and the anisotropy of dielectric constant of the chiral agent is within a range from 12 to 60.
A liquid crystal composition according to the third aspect of the present invention is a chiral nematic liquid crystal composition which contains a nematic liquid crystal mixture and a chiral agent, which exhibits a cholesteric phase at room temperature and which selectively reflects light of a specified wavelength, and the anisotropy of dielectric constant of the chiral agent is larger than that of the nematic liquid crystal mixture.
According to the first, the second and the third aspect of the present inventions, by adding a chiral agent with a relatively high anisotropy of dielectric constant to a nematic liquid crystal mixture, the anisotropy of dielectric constant of the obtained chiral nematic liquid crystal composition is larger than that of the nematic liquid crystal mixture, and thereby only a low driving voltage is necessary, or the anisotropy of dielectric constant of the obtained chiral nematic liquid crystal composition is large enough to necessitate only a low driving voltage. The necessity of only a low driving voltage permits use of inexpensive ICs, which results in a reduction in cost of a liquid crystal display.
The chiral agent to be added preferably has an anisotropy of dielectric constant within a range from 12 to 60. If the anisotropy of dielectric constant of the chiral agent is lower than 12, the effect of reducing the driving voltage is weak. On the other hand, if the anisotropy of dielectric constant of the chiral agent is larger than 60, the stability of the chiral agent is low, and there may be cased problems that the reliability of the liquid crystal display is not good and that the parts and members provided around the liquid crystal layer are dissolved.
In preparing a chiral nematic liquid crystal composition, if a nematic liquid crystal mixture is mixed with a chiral agent with a higher anisotropy of dielectric constant than the nematic liquid crystal mixture, the obtained chiral nematic liquid crystal composition will never has a lower anisotropy of dielectric constant than the nematic liquid crystal mixture. Thus, the problem that a high driving voltage becomes necessary due to addition of a chiral agent can be avoided.
A chiral nematic liquid crystal composition has a merit that the wavelength of light to be selectively reflected by the liquid crystal composition can be controlled by changing the content of the chiral agent therein. The content of the chiral agent is preferably within a range from 8 wt % to 45 wt % of the total of the nematic liquid crystal mixture and the chiral agent. If the content of the chiral agent is lower than 8 wt %, the chiral nematic liquid crystal composition may not be able to have a sufficient memory effect. On the other hand, if the content of the chiral agent is larger than 45 wt %, the chiral nematic liquid crystal composition may not exhibit a cholesteric phase at room temperature and/or may be solidified.
In the meantime, the anisotropy of dielectric constant of the chiral nematic liquid crystal composition is preferably within a range from 8 to 45. If it is smaller than 8, a high driving voltage will be necessary. On the other hand, if it is larger than 45, the reliability of the liquid crystal display will be worse.
Also, if such a chiral nematic liquid crystal composition contains two or more kinds of chiral agents, shifts of the wavelength of light selectively reflected by the chiral nematic liquid crystal composition due to changes in temperature can be adjusted, and the chiral nematic liquid crystal composition will have a stable temperature characteristic. Further, by adding a dye, the color purity of the wavelength of the reflection peak can be improved. Various kinds of well known dyes can be added, and dyes which are compatible with the liquid crystal composition are suited. For example, azo compounds, quinone compounds, anthraquinone compounds, etc. and dychroic dyes are usable, and combinations of such compounds can be also used. The content of the dye is preferably not more than 3 wt % of the total of the nematic liquid crystal mixture and the chiral agent. If the content of the dye is too large, the performance of selective reflection of the liquid crystal composition will be low, and the contrast will be low.
A reflective type liquid crystal display according to the fourth aspect of the present invention comprises, between a pair of substrate, at least one of which is transparent, a chiral nematic liquid crystal composition which contains a nematic liquid crystal mixture and a chiral agent, which exhibits a cholesteric phase and which selectively reflects light of a specified wavelength, and the anisotropy of dielectric constant of the chiral nematic liquid crystal composition is larger than that of the nematic liquid crystal mixture before being mixed with the chiral agent.
A reflective type liquid crystal display according to the fifth aspect of the present invention comprises, between a pair of substrate, at least one of which is transparent, a chiral nematic liquid crystal composition which contains a nematic liquid crystal mixture and a chiral agent, which exhibits a cholesteric phase and which selectively reflects light of a specified wavelength, and the chiral agent contained in the chiral nematic liquid crystal composition has an anisotropy of dielectric constant within a range from 12 to 60.
A reflective type liquid crystal display according to the sixth aspect of the present invention comprises, between a pair of substrate, at least one of which is transparent, a chiral nematic liquid crystal composition which contains a nematic liquid crystal mixture and a chiral agent, which exhibits a cholesteric phase and which selectively reflects light of a specified wavelength, and the chiral agent contained in the chiral nematic liquid crystal composition has a larger anisotropy of dielectric constant than the nematic liquid crystal mixture.
In each of the reflective type liquid crystal displays according to the fourth, the fifth and the sixth aspect of the present inventions, the anisotropy of dielectric constant of the chiral nematic liquid crystal composition is relatively large, and only a low voltage is necessary to drive the liquid crystal composition. Thereby, these liquid crystal displays can be produced at low cost.
Also, by using resin substrates, the liquid crystal displays will have merits of being light and thin and of being not fragile. Further, by providing spacers which are inorganic particles coated with adhesive resin, the gap between the substrates can be maintained evenly, and moreover, because the spacers are adhesive, a problem of display unevenness caused by movements of the spacers can be prevented.
It is preferred that the thickness of the liquid crystal composition between the substrates is within a range from 3 xcexcm to 10 xcexcm. If the thickness is less than 3 xcexcm, the reflection is low, and it is impossible to achieve a satisfactory display performance of a colored state. On the other hand, if the thickness is more than 10 xcexcm, a high driving voltage is necessary. Also, in this case, because the display performance of black is bad, the contrast is low.
Furthermore, by providing a plurality of polymer nodules between the substrates, it becomes possible to fabricate a large-scale liquid crystal panel. Also, due to these polymer nodules, the accuracy of the thickness of the gap between the substrates and the strength can be heightened. Further, the memory effect of the liquid crystal display can be improved.
Also, it is possible to provide a color filter instead of or in addition to adding a dye to the liquid crystal composition. For example, it is possible to provide a filter layer in the liquid crystal display. The material of the filter layer may be, for example, a material which is prepared by adding a coloring agent to a transparent substance or a material which is essentially colored. For example, the filter layer may be a thin film which is made of a specified substance which functions as a dye. The transparent substrate which is an element of the liquid crystal display can be replaced with such a filter material so that the same effect can be obtained.